(1). Field of the Invention
This invention relates in general, to signal communications cables, to their method of manufacture, and to connectors for connecting the ends of two such cables together. More particularly, the invention is concerned with communication cables capable of transmitting electrical signals, in particular, radio frequency and microwave signals, and optical signals, simultaneously. Further, the invention is concerned with means for interconnecting cable ends together.
(2). Description of the Prior Art
Communications between two points, whether across town, or around the world, depends upon the manipulation and controlling of signals within the electromagnetic spectrum. These signals are wavelike oscillations that can be described in terms of either length or frequency, i.e., the number of wave cycles per second, expressed as hertz (Hz). Ordinary alternating current from the power mains has a frequency of about 60 hertz, i.e., 60 cycles per second. High-frequency direct currents up to several gigahertz (1 gigahertz=1 billion hertz), however, are used in communications engineering and more particularly in radio communication.
Cables of the type designed for carrying high frequency radio signals (RF) and microwave signals have been known now for a number of years and have been constantly improved upon over the years. In general, such cables comprise one or more inner conductors i.e., a central conducting signal line, surrounded by a dielectric or insulating material, with an outer tubular-shaped conductor or shield surrounding the dielectric material and coaxial with the inner conductor. The outer conductor functions not only as a return electrode or ground but to protect the inner conductor signal from losses due to radiation and from outside disturbances. In some cases, a further protection layer or jacket is provided on the outside surface of the outer conductor, e.g., a tubular-shaped sheath of the same dielectric material or another plastic material offering greater abrasion resistance. The inner conductor or conductors and the outer conductor are made of an appropriate conductive metal, e.g. copper, stainless steel, silver, silver plated metals, aluminum, and various alloys. These cables are commonly referred to as "coaxial cables" and are used in various applications requiring a high frequency input. Examples include computer cables, community antenna and television (CATV) cables, and local area network (LAN) cables.
In U.S. Pat. No. 4,301,428, which issued on Nov. 17, 1981, there is disclosed a coaxial cable in which the inner conductor can be either a solid wire or a filament of glass on which has been coated a metallic or metal-like layer. This glass filament, apparently, is for the purpose of imparting its physical characteristics to the metallic layer. It is not disclosed, however, to have the characteristics of an optical layer.
Haverhill Cable & Manufacturing Corp., Haverhill, Mass. the assignee in this application, is a manufacturer of various miniature semi-rigid coaxial cables used to transmit electrical signals and which are characterized by their good flexibility and ability to be readily bent in tight environments. Such cables, in general, comprise a solid metal wire, circular-shaped, center or inner conductor surrounded by an outer, concentric seamless, tubular-shaped metal conductor with an annulus of a suitable dielectric or insulator separating the two conductors and maintaining that distance over the length of the cable. The purpose of semi-rigid coaxial cable is to transmit and/or receive a high speed, high-frequency microwave signal. Where great distances are involved, it is impractical because of cost to string a semi-rigid coaxial line; hence, an interconnection is used between semi-rigid and flexible cables. Coaxial cables manufactured by Haverhill Cable & Manufacturing Corp. can transmit and receive electrical signals over a wide frequency range, e.g., from about 60 cycles D.C. or A.C. to high frequency radio signals greater than 40 GHz (40 thousand million cycles). These latter high radio frequency cables find use in various applications, e.g., high speed computer systems, medical electronics, telecommunication systems, radar installations, and nuclear power plants. In componentry, semi-rigid coaxial cable is used, for example, in oscillators and amplifiers, printed circuit boards, and capacitor systems.
It is desired that the dielectric material used in such cables have not only acceptable insulating properties, i.e., a low dielectric constant, but low signal dissipation properties as well, i.e., that the dielectric material have a low inherent dissipation factor; otherwise, the dielectric material causes undesired attenuation of the electrical signal particularly at high radio (RF) and microwave operating frequencies of the cables. This power loss, which is sometimes referred to as "dielectric loss" contributes to the dissipation of the electrical signal. Efforts have been made over the years to improve the signal dissipation of coaxial cables, i.e., lessen the attenuation of the electrical signal, by selecting that particular dielectric material from known dielectrics so as to have the least dielectric loss properties in any particular cable construction and by creating and developing new dielectric materials, e.g., foam dielectrics from olefins and fluoropolymers. Thus, it is desired that any dielectric material used in a coaxial cable, particularly where such cable is to be used in microwave operating frequencies, be not only a good insulator but also be characterized by a low inherent dissipation factor. Moreover, the dielectric material desirably is either heat tolerant or characterized by high temperature operating properties.
The selection of any particular dielectric material for use in a coaxial cable for a specific application, however, depends not upon any one factor or characteristics thereof. Many considerations need be taken into account, in addition to those abovementioned, e.g., the end use for the particular cable, the desired overall size thereof, the operating frequencies, etc. Furthermore, consideration must be given as to the electrical conductors, e.g. the specific material of which the conductor is made, its conductivity, the relative sizes of the conductors and dielectric layer, and configuration. Thus, the design of a particular cable is not an exact science. It requires a great deal of empiricism.
For sometime now it has been desirable to transmit confidential information, military, business, or otherwise by means of a cable, rather than by radio or satellite transmission. Transmission of such information by these latter means is not secure, others desiring to do so being capable of intercepting the confidential information and acting on it, if so inclined. As a result, coaxial cable has been developed and used, in some cases at least, to transmit confidential information. Nevertheless, such coaxial cables are not entirely satisfactory, even though they may be run inside a special conduit, to mask the fact that such a cable exists. The conduit is not only relatively expensive, its very existence is believed indicative, to some at least, of a cable transmitting important or confidential information. One solution is to transmit confidential information in codes; however, this necessitates the use of expensive encoders and decoders.
Somewhat more recently optical fibers have been used in telecommunication cables by long distance telephone carriers. Accordingly, such telephone lines have been used to transmit various information, e.g., various data communications, business information, confidential information, etc. as optical signals. These hairlike fibers can transmit volumes of digitized information as pulses of laser light. At the receiving end, a photodetector senses the light pulses and translates them back into electrical signals to be routed as desired, In an optical fiber conductor, light travels along a fiber which is designed to confine the light to the interior of the fiber and allow it to follow the fiber's path, even around corners. Optical fibers, in general, comprise a central core of glass surrounded by a so-called clad, a similar material with a lower refractive index. Light pulsed through the fiber conductor is bent at the interface between the two materials toward the material with the higher refractive index--the core. Such fibers have been continually improved on over the years and can now transmit more than 95% of the light received for a distance of a kilometer. Nevertheless, the signal must be continually "repeated" over long transmission distances. The unit described "decibels (db) per meter" is used to measure how much of the light signal's strength is lost for every meter of transmission.
There are two kinds of optical fibers-multimode fibers and single-mode fibers. The latter are generally preferred as the glass core is extraordinarily narrow. Thus, the rays of light pulses have little space to bounce from side to side. As a result, the pulses of light retain their definition, permitting as many as thirty times the number of pulses/sec. to be transmitted as through a multimode fiber.
A further advantage of optical fiber transmission, compared to transmission over wire or cable, is that stray electromagnetic impulses do not effect glass as they do wires, so optical fibers are immune to errors in data or information transmission caused by electrical interferences. Thus, such transmission media is more suitable than wire or cable, in at least some cases, for transmission of highly important information such as military information. And, they offer tighter security because they are extremely difficult to tap, compared to wire transmission media. Even if such are tapped, that fact becomes readily known.
Others, heretofore, have disclosed combination or hybrid coaxial cables that comprise both electrical and optical conductors. Examples of this prior art are U.S. Pat. Nos. 4,158,478; 4,695,127; 4,867,527; and 4,896,939.
U.S. Pat. No. 4,158,478, which issued Jun. 19, 1979, discloses a coaxial optical fiber cable in which a centrally located electrical conductor of solid or several strands of copper is surrounded by an outer electrical conductor formed by one or more superposed braidings consisting of fine copper wires. These electrical conductors are separated from one another by a solid dielectric which according to the patentee may include polyethylene, polyethylene terephthalate and polystyrene used in compact or cellular form. A cladding of polyvinyl chloride surrounds the outer electrical conductor. A plurality of optical fibers surrounding the inner electrical conductor are embedded in the dielectric and are distributed symmetrically between the inner and outer conductors. While such a cable may be found satisfactory for simultaneous electrical and optical transmissions in certain applications, such a cable is not believed to be capable of transmitting electrical signals in the radio frequency range, particularly microwaves. Moreover, providing the optical fibers in surrounding, equally spaced-apart disposition about the central electrical conductor, embedded in the dielectric, presents certain problems in the handling of the optical fibers and the manufacturing of the cable. In any event, although the electrical conductors are coaxial with respect to one another, such are not coaxial with the optical fibers. The optical fibers, instead, define a circle which appears to be concentric, i.e., coaxial, with the inner conductor.
U.S. Pat. No. 4,695,127, discloses a hybrid coaxial-optical cable for concurrently carrying an electrical signal and an optical signal. The cable, as disclosed in that patent, includes a metallic conductor disposed at the center of the cable for carrying the electrical signal surrounded by a shield such as a metallic braid which is coaxial with the metallic conductor. The metallic braid is surrounded by a jacket of a tough, abrasion resistant thermoplastic material. The metallic conductor, in one aspect of the invention, is located within a buffer tube of somewhat rigid but resilient thermoplastic material surrounded by the metallic braid. The metallic conductor is held centered within the buffer tube, hence the shield, by means of a length of dielectric spacer wound around the conductor and concurrently engaging it and the inside circumferential surface of the buffer tube. Adjacent turns of the dielectric spacer are sufficiently spaced apart so that an optical conductor, which has an outside diameter much smaller than the inside diameter of the buffer tube, can also make a loose winding around the metallic conductor. In alternative constructions, optical fibers are located within a plurality of buffer tubes located within the metallic braid. These buffer tubes constitute centering means for the centrally located metallic conductor because each tube concurrently engages the metallic conductor and two adjacent tubes firmly to hold the electrical conductor centrally located in the cable. Optionally, a layer of metallic foil can be disposed under the braid and in contact therewith. Such a construction, according to the patentee, results in the lowest radio frequency leakage and lowest susceptibility to electrical noise. As disclosed in the patent, the braid functions to limit penetration of low frequency noise while the presence of the foil limits high frequency noise penetration. The hybrid cable is disclosed to exhibit relatively low capacitance because air space is left between the central metallic conductor and the braid coaxial therewith.
Although the hybrid cable disclosed in U.S. Pat. No. 4,695,127 may function quite well in some applications, its use is believed attendant with certain problems and disadvantages. First of all, the construction of the cable is relatively complex. Accordingly, its assembly, it is believed, is not that simple. Neither, it is believed, will the cable be economical to manufacture. Perhaps more importantly, however, such a cable is likely to be less bendable than desired in many applications involving the simultaneous transmission of radio frequency electrical signals and optical signals. Neither does the cable disclosed lend itself to the most compact construction desirable for many applications. Moreover, while the electrical conductors may be concentric, such are not concentric with the optical fiber conductor.
U.S. Pat. No. 4,867,527 discloses a combined electrical power and optical fiber cable which comprises a centrally disposed metallic conductor with insulation therearound, a sheath around the insulation, and a one or two part protective layer around the insulation. One or more tubes can be provided in a protective layer in each of which is located one or more optical fibers. The cables in the patent are disclosed to have substantially the same diameter as cables which are used only for conveying electric power. Although such a cable, as disclosed, may be found suitable in the simultaneous conveying of electrical power and transmission of optical signals, the cable cannot be used, it is believed, in the transmission of electrical signals, vis-a-vis electrical power, let alone those signals in the radio frequency, particularly microwave, range. Neither are the electrical conductors concentric with a fiber optic conductor so as to provide a truly coaxial cable offering the dual functions of transmitting electrical and optical signals simultaneously.
U.S. Pat. No. 4,896,939, which issued Jan. 30, 1990, to Donald G. O'Brien, discloses what is termed by that patentee, a hybrid fiber optic and electrical cable which comprises an optical fiber, a first tubular electrical conductor enclosing the optical fiber and a second tubular electrical conductor enclosing the first electrical conductor. A dielectric support element is disposed between the two electrical conductors for maintaining the conductors in coaxial relationship, with a selected uniform electrical impedance therebetween. The outer electrical conductor, in the preferred embodiment of the invention, is surrounded by an external protective layer of dielectric material. Further, there are disclosed connectors for coupling ends of the cable together. According to the patentee, the cable disclosed is capable of transmission of optical signals, wide bandwidth electrical signals, and DC or AC electrical power. While the signal communications cable disclosed in the patent may be capable of simultaneous transmission of optical and electrical signals, it is not believed capable of transmitting electrical signals in the high frequency, i.e., radio frequency particularly microwave frequency, range for various reasons. Nor was such, it is submitted, apparently even contemplated by the patentee. That patent makes no mention of the use of the cable disclosed therein at RF or microwave frequencies.
O'Brien discloses that the preferred dielectric material used in the cable construction is a tube or sheath of material such as polyethelene, a somewhat common dielectric material used in coaxial cables of relatively low operating frequency and at low operating temperature. Moreover, the patentee discloses that the outer electrical conductor can be a solid-walled tubular structure or, alternatingly, the outer conductor can be constructed from layers of conductive tape, braided or woven strands, or perforated conductive material. Such conductive materials are not really suitable for radio frequency signal transmissions. The braided shield cables are not able to carry electrical signals between two points without significant radiation loss, particularly at frequencies above 10 Hz. This is due, in part at least, to the imperfect contact between the woven, wire conductors. Such causes losses due to circulating currents.
The patentee, in U.S. Pat. No. 4,896,939, states however, that the cables disclosed therein are capable of "efficient transmission of optical signals, wide bandwidth electrical signals, and DC or AC electrical power." Nevertheless, the patent fails to define what is intended by "wide bandwidth electrical signals." It does not, it is believed, encompass high frequency, in particular radio frequency and microwave signals. If such did, no cable has been disclosed in the patent which could accomplish that purpose.
Simultaneous, multiple transmissions of messages over long distances are commonly accomplished by means of a carrier wave, i.e., a high-frequency current is modulated in various frequency ranges. For telephony, for example, a bandwidth of 3600 cycles/sec is adopted for each range and is adequate for the intelligible transmission of speech. Each range of this kind is comparable to a wire or cable and is called a channel. The frequency band, or bandwidth, of a telephony channel can, for example, be subdivided into 24 telephony channels, each of a lesser bandwidth than the 3600 cycles/sec., the bandwidth of a telephony channel. The same carrier wave can include various other electrical communication signals than telephony signals, each defined by other frequency ranges, and each signal having more or less greater bandwidths than the other. Thus, the term "bandwidth," has no definite meaning in and of itself. And, neither does "wide bandwidth." It can mean, for example, a wide bandwidth of a "transmission or carrier signal" or the wide bandwidth, i.e., the range of frequencies, occupied by any one information bearing signal that is only one of many signals accommodated by the carrier wave or transmission medium. The widebands encompassing radio frequency and microwaves and other high frequency media, such as fiber optics, can accommodate and transmit many communications, e.g., phone conversations, at one time, once the signals have each been translated to a higher frequency. This is accomplished, in general, by dividing the transmission or carrier path bandwidth, a wide bandwidth, whether wire or optical fiber, into several more narrow bands (or less wide bandwidths), each carrying, in some cases at least, a single communication. The greater the bandwidth, i.e., the wider the band, the more data that can be transmitted at any one time.
Thus, the patentee in U.S. Pat. No. 4,896,939 is believed to have contemplated an invention suitable primarily for electrical power transmission and, perhaps, the transmission of relatively low frequency electrical signals. The concern in U.S. Pat. No. 4,896,939 appears, nevertheless, to be with the transmission of electrical power to amplifiers or repeaters, for repeating the optical signal being transmitted at intervals along the conductor, to compensate for signal attenuation. Signal cables, however, are fundamentally different from power cables in both their intended use and in their design considerations and electrical properties.
Moreover, nothing is disclosed in that patent about the electrical or mechanical characteristics of the electrical signal conductors or of the optical characteristics of the optical fiber conductor. Furthermore, it is believed that any cable disclosed in that patent lacks the desired flexibility to be used in many applications for which the cables of the invention disclosed hereinafter will be found suitable.
Thus, there remains the need for a signal communications cable capable of transmitting electrical signals, particularly high frequency radio and microwave signals, and optical signals, simultaneously, of relatively simple and compact design, and with a minimum of electrical and optical signal attenuation.